COLLEVILLE-SUR-MER, France (Reuters) - France will never forget the sacrifice of the Allied troops who liberated it from Nazi Germany, President Emmanuel Macron said on Thursday, the 75th anniversary of the D-Day operation that helped bring World War Two to an end.

U.S. President Donald Trump and British Prime Minister Theresa May joined Macron at separate ceremonies along a 80km (50 mile) stretch of Normandy coastline, where more than 150,000 soldiers landed on June 6, 1944, under a hail of German fire.

"We know what we owe to you, our veterans: our freedom. On behalf of my country, I want to say 'thank you'," Macron told several dozen American D-Day combatants at a U.S. war cemetery overlooking Omaha Beach, one of five landing spots in Normandy.

"France will never forget."

Macron awarded the Legion d'honneur, France's highest award for merit, to five U.S. veterans and embraced each man warmly.

The Normandy landings were months in the planning and were kept secret from Hitler and his forces despite a huge trans-Atlantic mobilisation of industry and manpower.

Under the cover of darkness, thousands of Allied paratroopers jumped behind Germany's coastal defences. Then, as day broke, warships pounded German positions before hundreds of landing craft disgorged the infantry troops under a barrage of machine-gun fire and artillery.

Some veterans say the sea and sand turned red with blood during the operation. Dozens of U.S. rangers were felled by German machine-guns as they scaled the cliffs rising up from Omaha Beach to Colleville-sur-Mer, where the U.S. cemetery lies.

"You are among the very greatest Americans who will ever live," Trump said in his address, turning to the surviving veterans. "You are the pride of our nation, you are the glory of our republic and we thank you from the bottom of our hearts."

"These men moved through the fires of hell," he said. "They came here and saved freedom, and then they went home and showed us all what freedom is about."

People applauded as one of the veterans, 94-year-old Private Russell Pickett, rose shakily to his feet. "Tough guy," Trump said, before Macron helped lower Pickett back into his seat.

"SPECIAL GENERATION"

The Normandy landings remain the largest ever amphibious invasion and paved the way for western Europe's liberation.

Inaugurating a memorial to the 22,000 soldiers under British command who were killed on June 6, 1944, and in the ensuing battle for Normandy, British Prime Minister Theresa May saluted the bravery of the soldiers, many of whom were still boys when they waded ashore as shells screamed overhead.

"It's almost impossible to grasp the raw courage it must have taken that day to leap from landing craft and into the surf despite the fury of battle," May told a small gathering that included Macron and veterans, their uniforms laden with medals.

"These young men belonged to a very special generation ... whose incomparable spirit shaped our post-war world," she said.

The devastation wrought by two world wars in the first half of the 20th century fostered a decades-long era of cooperation between European capitals determined to protect their hard-fought peace, giving rise to what is now the European Union.

But even as Britain now tries to sever its ties with the bloc after four decades of membership, Macron told May some links between France and Britain were indestructible.

"Nothing will ever take away the links of spilled blood and shared values. The debates of the present in no way take away from the past."

An hour after sunrise, under clear blue skies, a lone piper on the remnants of an artificial harbour played Highland Laddie to mark the hour the first British soldier set foot on French sand. The Mulberry harbour was built to supply allied troops as they pushed the Germans back.

Restored wartime jeeps and amphibious vehicles lined the beach at Arromanches and in villages along the Normandy shore the flags of Britain, Canada and the United States, the main contributors to the Allied force, fluttered in the breeze.

The commemorations come against the backdrop of two years of forthright diplomacy and "America First" policymaking by Trump and his administration that have shaken the NATO alliance and tested relations with allies including Britain and France.

On the eve of the anniversary, France's president evoked the spirit of D-Day, saying: "These allied forces that together freed us from the German yoke, and from tyranny, are the same ones that were able to build the existing multilateral structures after World War Two.

"We must not repeat history, and remind ourselves what was built on the basis of the war," he said.